Dispatch communication systems using trunking are known. Such systems typically use frequency pairs (i.e., a transmit frequency and a receive frequency) that are assigned by a resource controller for transmitting and receiving messages. The resource controller assigns the frequency pair upon receiving a request for service from a communication unit. Upon receiving the assignment data the requesting unit and all target units tune to the appropriate frequencies and two-way communication can occur.
Dispatch systems, unlike cellular telephony, are typically single site systems. Where there are geographically adjacent dispatch systems no provision is typically made for call hand-off when a user leaves a service area. Where a target is initially in the service coverage area of a cell and subsequently passes into the service coverage area of an adjacent cell the adjacent cell typically will not provide service (forward transmitted messages) to the target unit. Where a requestor is in an adjacent cell, initially, the adjacent cell, again, will not provide service to the requesting unit.
In a geographic re-use context, cell sizes have gotten smaller to allow for more users in given geographic areas. The needs of dispatch systems, on the other hand, have not changed in terms of necessary service coverage area. As cells have gotten smaller a need exists to permit contacting target communication units within the service coverage areas of adjacent dispatch communication systems.